Very-High-Data-Rate Digital Subscriber Line (VDSL) is a bi-directional data transmission technology. Orthogonality of upstream versus downstream signals generally helps to mask echo. However, the orthogonality of downstream and upstream is difficult to maintain when the U0 band (e.g., 25 kHz-138 kHz) is utilized for data transmission to boost rate performance for longer loops. Thus, echo from upstream typically generates interference downstream in the customer side. Further, VDSL upstream and downstream typically employ a very wide-frequency bandwidth. Echo cancellation in such high bandwidth environments is generally very computationally expensive.
The International Telecommunications Union defines the Very-High-Data-Rate Digital Subscriber Line 2 standard (VDSL2) in a document entitled ITU-T G.993.2. Although this standard permits simultaneous reception of U0, U1, and U2 bands, it gives no technical details as to how such a reception may actually be implemented.